1. Field of the Invention
The present invention pertains to an apparatus for transporting a carriage along a track, substantially free of mechanical friction or magnetic drag, utilizing a hybrid support arrangement. The carriage is magnetically supported in a first direction, and the position of the carriage is stabilized in a second direction by passive means.
2. Description of Related Art
Transporting carriages along a track, substantially free of mechanical friction, has long been a goal. Numerous systems to transport passengers and cargo at high speeds or to convey manufactured articles or machine components in manufacturing systems have been devised. Prior art systems may be grouped as either xe2x80x9ccontacting systemsxe2x80x9d, i.e., wherein the carriage is in mechanical contact with the track, or xe2x80x9cnon-contacting systemsxe2x80x9d, i.e., wherein there is no mechanical contact between the carriage and the track. Each group of systems typically suffers from certain disadvantages.
Contacting Systemsxe2x80x94Conventional mechanical track systems which require mechanical contact between the carriage and the track typically employ either slide mechanisms or wheels to transport the carriage. Such systems suffer from significant mechanical friction. Slide mechanisms, which require the use of either hydrostatic or hydrodynamic lubrication, almost always leak and contaminate the equipment. Wheeled systems employ low-friction ball or roller bearings, which also typically require lubricants. Although bearings in wheeled systems may incorporate seals to contain the lubricant, the rolling contact of the wheel with the track usually generates wear particles which ultimately contaminates the equipment.
Non-contacting Systemsxe2x80x94Conventional non-contacting transport systems include either aerostatically supported arrangements, commonly referred to by the terms xe2x80x9cair-bearing systemsxe2x80x9d or xe2x80x9cgas-bearingsxe2x80x9d, or magnetically supported arrangements, commonly referred to by the terms xe2x80x9cmagnetic bearingsxe2x80x9d or xe2x80x9cmagnetic levitation systemsxe2x80x9d.
Aerostatic arrangementsxe2x80x94Aerostatic or air-bearings use a thin film of a high-pressure gas, typically air, to support a load. Since gases have very low viscosity, the gaps between elements in such bearings must be small, typically less that about 10 micrometers. In typical linear aerostatic systems, the gas is provided to the moving element or carriage. This restricts the range of motion of the carriage due to the need to supply the high-pressure gas to the carriage from a typically stationary gas source. In such linear aerostatic arrangements, one element is a pad, through which the high-pressure gas is supplied, and the other element is a slide or slideway, on which the pad is supported by the film of gas. The pad may be perforated, slotted, or made of a porous material to distribute the gas over the face of the pad. In such arrangements, the extremely small clearances require that the surface of the pad closely conform to the slideway. This is typically accomplished by making the both the pad and the slideway extremely flat. Even when the pad is articulated to permit it to follow the surface of the slideway, only very gradual curves of very large radius can be accommodated.
Magnetically Supported Arrangementsxe2x80x94Numerous magnetic levitation arrangements using permanent magnets and electromagnets, or combinations of the two, are known. British Patent 867,045 and British Patent 1,035,764 are typical examples of such prior art arrangements.
In electromagnetic field theory, it has long been known that magnetic levitation arrangements, using only permanent magnets, cannot be simultaneously stable in three orthogonal directions. Earnshaw first published his findings in 1849 and his work is popularly known as xe2x80x9cEarnshaw""s Theoremxe2x80x9d. The term xe2x80x9cmagnetic stabilityxe2x80x9d is usually defined in mathematical terms using the convention of a Cartesian coordinate system, i.e., orthogonal x, y, and z coordinates. For a system having a magnetic restoring force F and a displacement along the y-axis, xe2x80x9cmagnetic stabilityxe2x80x9d means that the derivative of the magnetic restoring force F with respect to the displacement direction shall be negative, i.e., for a displacement along the y-axis, dFy/dy less than 0. Since Earnshaw""s Theorem requires that the sum of the force derivatives be equal to zero, i.e., dFx/dx+dFy/dy+dFz/dz=0, it can be seen that all three derivatives can not simultaneously be negative.
Active stabilization systems employing position sensing and feedback control have been employed to overcome the limitations imposed by Earnshaw""s Theorem. A prior art system, such as that described in U.S. Pat. No. 4,142,469, is typical of an actively stabilized magnetic levitation arrangement. This patent discloses a tracked vehicle system employing a combination of permanent magnets and one or more electromagnets, the electromagnets being energized by the feedback control system. The electromagnets are employed to control the magnet flux, which controls the lifting force, and they are used to maintain lateral position control for tracking the vehicle along the desired path.
FIG. 1 shows a system 2 from which the principles underlying an actively stabilized magnetic levitation arrangement may be appreciated. Permanent magnets 3 for a carriage 4 and permanent magnets 5 for a track base 6 levitate the carriage 4 in a first direction. The position of the carriage 4 is stabilized in a second direction by the use of carriage position or gap sensors 7 and an active carriage position feedback mechanism to energize one or more electromagnets 8. An active stabilization system, as shown in FIG. 1, results in increased complexity and cost of the levitation system.
Prior art magnetically levitated transport systems typically suffer from a significant amount of magnetic drag. Magnetic drag, while somewhat analogous to mechanical friction, changes in magnitude as the carriage speed changes. Additional power must therefore be supplied by the drive system to overcome the magnetic drag, as well as the aerodynamic drag which results from air resistance.
U.S. Pat. No. 5,809,897, issued on Sep. 22, 1998, discloses an electromagnetic induction ground vehicle levitation guideway for a vehicle having magnets for providing magnetic levitation of the vehicle. The vehicle is adapted to travel in a longitudinal direction along the guideway. The guideway comprises a beam support member for supporting the weight of the vehicle, and breakaway energy absorption structure mounted to the beam support member for absorbing kinetic energy from the magnetic levitation vehicle in the event of loss of magnetic levitation.
U.S. Pat. No. 5,388,527, issued on Feb. 14, 1995, discloses a multiple magnet apparatus for positioning a magnetic levitation ground vehicle that travels along a guideway at a selected position along an axis of perturbation relative to the guideway. The vehicle carries a first magnet, having its poles aligned perpendicular to the travel path of the vehicle and to the axis of perturbation, and second magnet, with its poles aligned parallel and opposite to the poles of the first magnet. The second magnet is adjacent to the first, and spaced away along the axis of perturbation. The guideway may carry conductors to interact with the vehicle magnetic fields to maintain the vehicle at the vertical position. The conductor may be a ladder, discrete coils, or a helical meander winding. The conductors may be oriented either vertically or horizontally, depending on whether the positioning device is used for suspension, or guidance.
U.S. Pat. No. 5,440,997, issued on Aug. 15, 1995, discloses a magnetic suspension transportation system for a vehicle/rail transportation system, where interacting sets of magnets are positioned on the vehicle and the rail to suspend the vehicle from the rail and permit low friction, non-contacting movement along the rail. Also, laterally facing air castors are provided for lateral support. The transportation system is stabilized in two directions by magnetic means, and uses a passive stabilization means with air pressure for stabilizing the position of the vehicle in a third direction.
The present invention overcomes the disadvantages of the prior art by utilizing a passive stabilization means to overcome the limitations imposed by Earnshaw""s Theorem. As used in the context of the present invention, the term xe2x80x9cpassive stabilization meansxe2x80x9d means an arrangement for stabilizing the position of the carriage in the magnetically unstable direction without the use of carriage position sensors, without a servo-controlled electromagnet, or any feedback mechanism. The present invention provides a stable carriage suspension arrangement that has the combined advantages of imposing no mechanical friction on the carriage, and significantly reducing or even eliminating magnetic drag, with the simplicity of no active stabilization arrangement.
The present invention describes an apparatus for transporting a carriage along a track in a non-contacting, magnetically drag-free manner. The apparatus is described in reference to a set of Cartesian coordinates, i.e., orthogonal x, y, and z coordinates. The apparatus comprises a track oriented in a direction along the x-axis, a carriage guided by the track, and a motor for propelling the carriage along the track. One or more permanent magnets are mounted on the carriage, each carriage magnet having its magnetization vector oriented in the yz-plane at a predetermined angular direction relative to the z-axis. One or more permanent magnets are mounted on the track, each track magnet having its magnetization vector oriented in the yz-plane at a predetermined angular direction relative to the z-axis. The track magnets may be either arranged in a single row along the x-axis or arranged in pairs in two rows that are disposed on opposite sides of the x-axis, each row of track magnets being displaced from the x-axis in the z-axis direction. The choice of individual track magnets or pairs of track magnets depends upon the track configuration.
The track magnets interact with the one or more carriage magnets to produce either an attractive or repulsive interaction that is magnetically stable in a first direction in the yz-plane and is magnetically unstable in a second direction in the yz-plane. In a first embodiment of the invention, the direction of magnetic stability is along the y-axis and the direction of magnetic instability is along the z-axis. In a second embodiment, the direction of magnetic stability is along the z-axis and the direction of magnetic instability is along the y-axis.
In the first embodiment, the magnetic support is attractive, and a passive stabilization means is employed to stabilize the position of the carriage in the magnetically unstable z-axis direction by substantially constraining the motion of the carriage to the xy-plane, without inducing mechanical friction or magnetic drag on the carriage. This first embodiment enables the carriage to be transported in a stable path along the track without mechanical contact while supporting a load applied in the y-axis direction. When the carriage supports a y-axis direction load, the path of the carriage is displaced or offset from the x-axis in the y-axis direction at a distance corresponding to the magnitude of the applied load and the y-axis direction xe2x80x9cmagnetic stiffnessxe2x80x9d of the magnetic support. The y-axis direction stiffness of the magnetic support is a function of the relative position and orientation of the carriage and track magnets, as will be discussed. When the carriage supports a z-axis direction load, the path of the carriage is displaced or offset from the x-axis in the z-axis direction at a distance corresponding to the magnitude of the applied load and the z-axis.direction stiffness. Depending on the type of stabilization means employed, the z-axis direction stiffness is typically quite different from the y-axis direction magnetic stiffness.
In the second embodiment, the magnetic support is repulsive, and a passive stabilization means is employed to stabilize the position of the carriage in the magnetically unstable y-axis direction by substantially constraining the motion of the carriage to the xz-plane. This second embodiment enables the carriage to be transported in a stable path along the track without mechanical contact while supporting a load applied in the z-axis direction. When the carriage supports a z-axis direction load, the path of the carriage is offset from the x-axis in the z-axis direction at a distance corresponding to the magnitude of the applied load and the z-axis direction xe2x80x9cmagnetic stiffnessxe2x80x9d of the magnetic support. The z-axis direction stiffness of the magnetic support is a function of the relative position and orientation of the carriage and track magnets, as will be discussed. In a manner similar to the first embodiment, the y-axis direction stiffness depends on the type of stabilization means employed, and is typically quite different from the z-axis direction magnetic stiffness.